This is found all over the ancient world. The older stuff underneath is much much more advanced and difficult than all the stuff built over the top of it. How could that happen. it is completely backwards from all present narratives and misguided logic.
Atlantis? Fallen civ? Aliens?
No it isn’t.
Here’s how it could happen, if you read this article, even he makes reference to historical periods during which the level of workmanship had declined from earlier periods.
Many great empires and cities lie beneath the dust. For some period of time Rome was being used to pasture sheep.